1. Field of the Invention
The present invention relates to a method of manufacturing a circuit board in which a copper member is directly bonded to a substrate made of ceramics.
2. Description of the Related Art
In the Published Unexamined Japanese Patent Application No. 60-32343, a method of manufacturing a circuit board in which a copper member is bonded to a substrate made of ceramics such as an aluminum nitride (AlN) sintered body is disclosed. In this method, a paste or foil of Ti or Zr serving as an active metal is interposed between the substrate and the copper member, and the substrate and the copper member are heated in a vacuum atmosphere to be bonded to each other. In this method, however, the paste must be coated, or the foil must be cut into pieces having sizes of the substrate and the copper member. In addition, since the substrate and the copper member must be heated in a vacuum atmosphere to be bonded to each other, a batch-type heating furnace must be used. For this reason, the process becomes cumbersome to degrade mass productivity.
In the Published Unexamined Japanese Patent Application No. 59-3077, a method of manufacturing a circuit board in which a copper member is directly bonded to a substrate made of an AlN sintered body is disclosed. In this method, the substrate is oxidized on its surface, and a copper member containing 100 to 2,000 ppm of oxygen is brought into contact with the oxide layer formed on the substrate. Thereafter, the resultant structure is heated in an inert gas atmosphere such as nitrogen gas or Ar gas, and the copper member is directly bonded to the substrate, thereby manufacturing a circuit board. In this method, since the heating is performed in the inert gas atmosphere, although the copper member contains oxygen, a hypoeutectic alloy liquid phase is partially decomposed on the bonding interface, and oxygen is compositionally decreased. The equilibrium oxygen partial pressure of a reaction 2Cu.sub.2 O.revreaction.4Cu+O.sub.2 at a temperature of 1,065.degree. C. is 1.5.times.10.sup.-6 atm., and the reaction is expected to be performed in the hypoeutectic alloy liquid phase. Namely, the hypoeutectic alloy liquid phase is partially decomposed due to heating in the inert gas. In addition, since the heating temperature for bonding the copper member and the substrate is higher than a temperature corresponding to a eutectic line in a hypoeutectic region, an equilibrium oxygen partial pressure at this temperature is further increased. As a result, a eutectic alloy liquid phase produced in the copper member by reduction which the reaction moves forward right, is decreased. Therefore, only a very small amount of eutectic alloy liquid phase is produced on the bonding interface, and the eutectic alloy liquid phase cannot be used for bonding the copper member and the substrate. As a result, a portion where the copper member is not microscopically bonded to the substrate is made, or the bonding strength of a circuit board is disadvantageously decreased.
In the Published Unexamined Japanese Patent Application No. 59-3077, a method in which a member made of copper containing no oxygen (oxygen-free copper) is used is disclosed. In this method, a substrate made of an AlN sintered body and the oxygen-free copper member are heated in a reactive gas atmosphere containing 0.03 to 0.1 vol % of oxygen to be bonded to each other. However, even in this method, the following problem is posed. That is, since the oxygen content is as large as 0,03 to 0.1 vol % in the heating atmosphere, oxidation is difficult to properly control, and the surface of the copper member is excessively oxidized. As a result, the oxide layer must be removed from the surface of the copper member by etching or the like after the bonding step, or the oxide layer must be removed by reduction in a hydrogen atmosphere. When these steps are employed, the number of steps is increased to degrade mass productivity, and the surface of the copper member is roughened.
In the specification of the British Patent Publication No. 761045, a method of manufacturing a circuit board in which a copper member is bonded to a substrate made of an aluminum oxide (alumina) sintered body is described. In this method, the copper member is oxidized in advance, an oxide layer formed on the copper member is located on the substrate of the alumina sintered body, and the resultant structure is heated at a temperature which is higher than the melting point of copper (1,083.degree. C.) and lower than the melting point of cuporus oxide (about 1,200.degree. C.). This specification describes that copper is melted, but cuporus oxide is not melted upon the heating operation. Therefore, the substrate and the copper member are bonded to each other in the region having an amount of oxygen larger than that of a eutectic point (a point of 3,900 ppm of oxygen) in the Cu-Cu.sub.2 O phase diagram. Since the copper member is inevitably oxidized, when the copper member is applied to a circuit board, the oxide layer must be removed from the copper member, thereby roughening the surface of the copper member.
In the specification of the U.S. Pat. No. 3,744,120, the following method is disclosed. That is, in this method, a copper member is arranged to a substrate made of an alumina sintered body, and the copper member and the substrate are heated in a reactive gas atmosphere containing 0.01 to 0.5 vol % of oxygen to be bonded to each other. In this method, however, since the heating operation is performed in the reactive gas atmosphere containing 0.01 to 0.5 vol % of oxygen, an oxide layer is formed on the surface of the copper member. Therefore, a cumbersome step is required to remove the oxide layer, and the surface of the copper member is roughened.
In the Published Examined Japanese Patent Application No. 60-4154, another method of bonding a copper member to a substrate made of an aluminum sintered body is disclosed. In this method, after the surface of the copper member is oxidized, an oxide layer of the copper member is arranged on the substrate, and the copper member and the substrate are heated in an inert gas to be bonded to each other. In this method, as in the method of the above-described Published Unexamined Japanese Patent Application No. 59-3077, a eutectic alloy liquid phase serving as a sintering agent is decomposed at the bonding temperature (about 1,070.degree. C.), and the eutectic alloy liquid phase is decreased by reduction. As a result, the eutectic alloy liquid phase i not supplied to the entire surface of the substrate made of the alumina sintered body at the bonding temperature, and a portion where the copper member and the substrate are not microscopically bonded is made. In the Published Examined Japanese Patent Application No. 60-4154, the following method is described as a method of supplying a binder in addition to the method of oxidizing the copper surface. That is, particle-like compound made of a binder and copper may be applied to the bonding surface of the copper member. However, even when the particle-like compound is interposed between the substrate and the copper member, since the substrate and the copper member are bonded to each other in an inert gas at an equilibrium oxygen partial pressure, reduction naturally occurs. In addition, since use of the binder is required, the step of bonding the copper member and the substrate is cumbersome.